Dry eye syndrome (“dry eye” for short) refers to any one or more deficiencies of the tear film which coats the surfaces of the cornea and conjunctiva of the eye. The tear film is a layer of substances secreted by glands around the eye and is intended to keep the eye surfaces constantly lubricated. The tear film protects the corneal tissue, keeps the eye comfortable, provides proteins and nutrients for the eye, and refracts light. Thus, dry eye generally involves discomfort, pain, poor vision, and/or eye injury. Common symptoms described by patients include scratchiness or grittiness, sensation of a foreign body in the eye, aching, burning, fatigue, contact lens discomfort, excess mucus discharge, sensitivity to light, sensitivity to smoke, and blurred vision.
Most clinicians diagnose and treat dry eye based on the symptoms alone. For example, the patient may fill out a questionnaire that is scored according to a point scale or otherwise evaluated by the clinician to determine whether a diagnosis of dry eye is indicated. This subjective methodology fails to provide objective evidence of tear film deficiency.
Various tests have been developed to measure tear film and determine tear film deficiency in a more objective manner. One such test is the Schirmer test, which is intended to measure the amount of tears that are produced by the eye. If the tears are collected for some time, for example five minutes or so, then one can determine whether the amount produced is sufficient for maintaining eye health. If total tear production is below normal, this is evidence of dry eye. If total tear production is normal or above normal, but there are still symptoms of ocular discomfort, then evaporative dry eye, for example due to blepharitis or Rosacea, may be present. In the Schirmer test, a 35 mm×5 mm size filter paper strip is used to measure the amount of tears that are produced over a period of five minutes. The strip is placed at the junction of the middle and lateral thirds of the lower eye lid, and is conducted tinder ambient light. The patient is instructed to look forward and to blink normally during the course of the test. A negative test (more than 10 mm wetting of the filter paper in five minutes) means the patient produces a normal amount of tears. Patients with dry eye have wetting values of less than 5 mm in five minutes. An important limitation of the Schirmer test is that there may be considerable variability in the results of tests done at different times and by different doctors. So, although this is perhaps the most common dry eye test performed, its main utility may really be in diagnosing patients with severe dry eye. Sequential tests to follow the course of a patient with mild dry eye may not be of value. Moreover, there is one point of some debate in the Schirmer test. When an anesthetic eyedrop is not used, then this test is thought to measure the basal tear secretion plus the reflex tear secretion. When an anesthetic eyedrop is used, then this test is thought to measure only the basal tear secretion. There is compelling reason to believe that the tears measured by these two different methods may not sufficiently differentiate between basal and reflex tear production. Most clinicians perform this test after using anesthetic eyedrops to numb the eye. However, The National Eye Institute workshop on dry eyes recommended not to use anesthetic eyedrops before performing this test. The cutoff value is similar whether or not anesthetic is used. To measure the reflex tear secretion, the Schirmer II test may be performed. The Schirmer II test is performed by irritating the nasal mucosa with a cotton-tipped applicator prior to measuring tear production.
Many clinicians regard the Schirmer test as unduly invasive and of little value for mild to moderate dry eyes. Other less invasive methods to assess the adequacy of tear production have been developed. The phenol red thread test is one such test and is commercially available under the trademark ZONE-QUICK®. A cotton thread impregnated with phenol red dye is used; phenol red is pH sensitive and changes from yellow to red when wetted by tears. A crimped end of a 70 mm long thread is placed in the lower conjunctival fornix. After fifteen seconds, the length of the color change on the thread—indicating the length of the thread wetted by the tears—is measured in millimeters. Wetting lengths should normally be between 9 mm and 20 mm. Patients with dry eye have wetting values of less than 9 mm.
Another test is the tear breakup time (BUT) test. Normal tear film is continuous, and blinking maintains the tear film continuity. If the eye is kept open long enough, without blinking, the tear film will start to break up. As a result, the eye will feel uncomfortable causing the patient to blink. In patients with dry eye the tear film is unstable, and breaks up faster. Therefore, the tear break up time in patients who have dry eyes is shorter. Said in another way, if the patient's tear break up time is relatively short then the patient may have dry eye. Fluorescein break up time (FBUT) is used most commonly. A strip of fluorescein is applied in the lower eyelid fornix and then removed. The patient is asked to blink three times and then look straight forward, without blinking. The tear film is observed under cobalt-blue filtered light of the slitlamp microscope and the time that elapses between the last blink and appearance of the first break in the tear film is recorded with a stopwatch (a break is seen as a dark spot in a sea of blue). FBUT of ten seconds or less is consistent with dry eye. FBUT has important limitations. For example, touching the filter paper strip to the conjunctiva can stimulate reflex tearing. Although special fluorescein strips designed specifically for FBUT are available and are claimed to deliver a fixed microvolume of fluorescein without stimulating reflex tearing, the mere presence of fluorescein in the tears perhaps also changes the tear film properties, so FBUT measurements may not be truly physiological.
To overcome these limitations, non-invasive break up time (NIBUT) methods have been developed. They are called non-invasive because the eye is not touched. Instruments such as a keratometer, hand-held keratoscope or tearscope are required to measure NIBUT. A prerupture phase that precedes actual break up of the tear film can also be observed with some techniques. This pre-rupture phase is termed tear thinning time (TTT). Measurement is achieved by observing the distortion (TTT) and/or break up (NIBUT) of a keratometer mire (the reflected image of keratometer grid). The clinician focuses and views the crisp mires, and then records the time taken for the mire image to distort (TTT) and/or break up (NIBUT). NIBUT measurements are longer than fluorescein break up time. NIBUT values of less than 15 seconds are consistent with dry eye. TTT and NIBUT are considered to be more patient-friendly, repeatable and precise than the methods discussed above, but like the other methods they are time consuming and require skill to administer.
Thus, a need exists for a diagnostic tool capable of quickly and objectively measuring tear film without significant discomfort to the patient, and without the need for significant skill and practice on the part of the person administering the test.